In many wireless communications systems such as a multiple access spread spectrum OFDM system, where the frequency band is subdivided into a set of tones, e.g., sub-carriers, it is desired for the wireless terminal devices, e.g., mobiles, to convert directly, e.g., without an IF stage, from received RF to baseband in their receivers and directly from baseband to RF in their transmitters. As part of the conversion process, generally, some noise is introduced at the DC tone, e.g., self-interference. This noise is typically device dependent, generally a constant for a given device's receiver or transmitter, and may vary slowly, e.g., as a function of device temperature. This DC tone noise, if not compensated for, tends to degrade signaling performance. In some known applications, the DC tone is simply not used to convey information; however, such an approach wastes the air link resource.
One known approach to measuring and compensating for a mobile receiver's self-interference characteristics is to temporarily disconnect its input, perform test measurements, derive a compensation value from data collected during the suspension interval, and then use the compensation value as a correction. The testing interval can be at initialization and/or at subsequent intervals during operation. This approach has the disadvantage that it suspends the ability of the mobile to receive and process information on the complete set of downlink tones during the testing intervals thus increasing overhead and reducing throughput. This approach is particularly inefficient in embodiments where the complete set of downlink tones includes many additional tones in addition to the downlink DC tone. If the testing is limited to mobile initialization, then changes in the receiver due to thermal characteristics are not well compensated. On the other hand, periodic suspension of downlink signaling reception during operation is undesirable since it limits throughput capacity, may result in missed broadcast or assignment signals, introduces delays in paging, disrupts control loop signaling, and/or introduces disruptions in user data forwarding in communications sessions.
In the uplink, a base station is typically receiving uplink signals from a plurality of mobiles. At any one given time, one of the mobiles can use the uplink DC tone, and at different times, different mobiles may use the uplink DC tone. Each of the mobiles typically have different transmitter DC tone noise characteristics. In some embodiments, where a given mobile is assigned to use a set of uplink tones for a number of consecutive symbol time intervals, one approach to noise removal on the uplink is to include a predetermined fixed reference modulation signal conveyed by each tone of the set of allocated tones. For example, in an embodiment where a mobile is allocated a set of uplink tones to use for seven consecutive symbol intervals, the fourth symbol interval may be used to convey a fixed predetermined reference modulation symbol on each of the allocated uplink tones. The base station receives the reference modulation symbol for each tone, determines a difference from the expected value, and determines compensation values to use for the each tone. This approach has the disadvantage that it includes a relatively high amount of overhead for each of the uplink tones of the system, thereby reducing throughput significantly. This approach also does not take into account that the received DC tone has a different noise characteristic than the other tones of the system. In addition this approach does not take into account that the received measured modulation symbols may significantly change from one symbol interval to the next as a result of airlink interference variations. Using a single received reference symbol per tone for a set of consecutive symbol time intervals provides a poor correction value if the airlink interference happens to deviate from the average airlink interference during the single interval used to convey the reference modulation symbol. In addition the reference modulation symbol information, e.g., transmission power level used, needs to be controlled tightly by the wireless terminal, and known to the receiving base station.
In view of the above discussion, there is a need for improved methods and apparatus which provide means to measure and compensate for DC tone noise. Methods and apparatus that include special DC tone processing without interfering with the signaling on the other (non-DC tone) tones would be beneficial. Improved methods and apparatus that limit the amount of overhead used to achieve the DC tone noise characteristic compensation would be also beneficial. Methods and apparatus that provide for measuring of the DC tone characteristics during mobile operation, thus providing adjustments for thermal variations, without significantly disrupting in process communications sessions and/or control operations would also be useful. Methods and apparatus of measuring and compensating for uplink DC tone noise that are adapted to take into consideration: the variation in uplink DC tone assignment to different mobiles, the air link noise introduced, and the variation in air link noise from one symbol time to the next would be useful.